Add avatars-gravatar plugin to the checks pipeline

Change-Id: I576fa428f136d2b2cf2c8322ba2e963c6f6c5f20

zuul.d/projects.yaml

diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index ceb258f..8cbceb4 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -93,6 +93,12 @@
         - gerrit-plugin-build
 
 - project:
+    name: plugins/avatars-gravatar
+    check:
+      jobs:
+        - gerrit-plugin-build
+
+- project:
     name: plugins/batch
     check:
       jobs: